This invention relates to a process for the preparation of olefin polymers, in particular a multi-stage process in which hydrogen and a metallocene or other single site catalyst are present in the reaction mixture in at least one of the earlier polymerization stages, as well as to olefin polymers produced thereby.
In the preparation of olefin polymers it is known to use a variety of catalyst systems, e.g. Ziegler Natta catalysts, metallocene catalysts, chromium catalysts, and chromocene-silica catalysts as well as to perform the polymerization in one or more stages, e.g. in two or more reactors arranged in series. Typically such reactors may be gas phase or slurry phase reactors or a combination of slurry phase and gas phase reactors.
One of the reasons for using multistage polymerization reactions has been to produce a final polyolefin product which has a broad, bimodal or multimodal molecular weight distribution and which as a result has improved processability (see for example WO92/15619 (Phillips Petroleum)).
In WO92/15619 there is described a process for preparing bimodal polyolefins by a two stage polymerization in which in the first stage a relatively higher molecular weight copolymer is produced and in which in the second stage hydrogen is present and a relatively lower molecular weight homopolymer is formed. The catalyst used in the first and second stages is a metallocene or a mixture of metallocenes.